Since its discovery, the ability of x rays to penetrate through material has been exploited extensively, such as in non-invasive and non-destructive imaging in medical and industrial applications.
For example, a number of manufacturers have developed imaging tools for failure analysis of integrated circuit (IC) packaging that take advantage of the penetrating power of hard x rays with tens to hundreds of kilo electron-Volts (keV) energy. These x-ray inspection tools typically provide resolutions on the order of tens of micrometers. This is sufficient for inspecting most large features in IC packages. They provide sample mounting mechanisms and multi-axis motion control to allow the operator to translate, zoom, and tilt the sample with a joystick while observing the image in real time. These systems are widely deployed in packaging failure analysis (FA) labs.
In addition, with increasing feature density and complexity, two-dimensional (2D) x-ray micrographs tend to contain too much overlapping information for the operators to interpret, necessitating the inclusion of a quantitative third dimensional (3D) imaging capability in the FA tools.
Many x-ray inspection tool manufacturers have developed systems to meet the IC manufacturer's needs. The following is a brief overview of these x-ray inspection tools.
Some current commercial tools are arranged in a relatively simple projection geometry, in which the radiation produced by an x-ray source is allowed to penetrate the sample, and the transmitted radiation is collected by the detector. With this setup, the geometrical magnification of the system is:
                              M          =                                                    L                s                            +                              L                d                                                    L              s                                      ,                            (        1        )            
where, Ls is the source to sample distance and the Ld is the sample to detector distance. As a result, the achievable resolution of these systems can be derived, being roughly:
                              δ          ⁢                      >            _                    ⁢                                                    M                -                1                            M                        ⁢            s                          ,                            (        2        )            
where s is the size of the x-ray source spot.
From this relationship, in order to achieve high resolution, one should make M as close to 1 as possible. In the limit where M=1, the sample plane overlaps the detector plane, which is the geometry of contact printing mode.
Contact printing, however, requires a detector with high enough resolution to sample the contact image, since with the magnification close to one. Thus, the detector resolution must be on par with the sample feature to be resolved.
In the absence of such a high resolution detector, an alternative imaging mode is to increase the geometric magnification so that the features in the sample are magnified sufficiently such that they can then be sampled with a detector with coarse resolution. In this projection mode, M>1, the resolution is about the same as the source size. Thus, high resolution requires a source with very small spot size.